Plate heat exchangers of this kind are well known and are utilized in many different connections.
Sometimes it is desired that part of a liquid to be heat treated is by-passed a plate heat exchanger. In such a case a tubular connection is usually connected outside the plate heat exchanger between its inlet and its outlet for said liquid, a valve being arranged to direct a larger or smaller part of the liquid arriving at the plate heat exchanger through said tubular conduit. If desired, a valve of this kind may be adapted to direct the whole of the liquid flow through either the plate heat exchanger or the conduit. An arrangement of the kind just described is shown for instance in EP-O 122 133-B1.
A conduit of the kind just described requires, however, a relatively large space outside the plate heat exchanger and brings with it additional costs for its connection to the plate heat exchanger inlet and outlet conduits.
It is previously known, e.g. through EP-O 316 510-B1, to make arrangements within a plate heat exchanger for a by-pass flow of a certain part of the liquid supplied to the plate heat exchanger. However, in such a known arrangement a separate casing surrounding the package of heat exchanging plates is necessary. This means that the plate heat exchanger becomes substantially more expensive than if the casing could be dispensed with. In the known arrangement according to EP-O 316 510-B1 it is further impossible to pump said liquid into the plate heat exchanger by a high pressure, since the by-passing of part of the liquid is intended to take place through an overflow outlet within said casing.